Habitability of the Paleo-Earth as a Model for Earth-like Exoplanets

The Phanerozoic is the last Eon of Earth history, from 542 million years ago to today. This was the period when large and complex life started to populate the ocean and land areas of our planet (Figure 1). During the Phanerozoic our planet became more habitable and this opened the opportunity for life to evolve and spread globally, specially in land areas. Certainly, there were periods more habitable than others within this Eon as Earth evolved. Future observations of Earth-like exoplanets (mesoplanets) might reveal them at any stage, from desert to forest planets, and understanding Earth's past might help to interpret their potential for life. Therefore, in this analysis we evaluated the habitability of Earth during the Phanerozoic as a model for comparison for future observations of Earth-like exoplanets.

Figure 1. Timeline of the last 700 million years of Earth, a period that includes the late Proterozoic (2500 Ma to 542 Ma) and the Phanerozoic (542 Ma to today).

Habitability refers the quality of an environment for life and it is usually correlated with the presence of life. The habitability of our planet is controlled by a complex interaction of biogenic gases (particularly carbon dioxide, oxygen, and nitrogen), the availability of water (both as liquid and vapor), temperature, and sunlight. For primary producers like plants on land, temperature and water availability are the most important factors controlling their distribution. The distribution of phytoplankton in the oceans is controlled by temperature too, however it is not limited by water availability but by nutrients. Both temperature and water availability are more important factors at seasonal scales, while oxygen, carbon dioxide, nitrogen and sunlight are more relevant at much longer time scales.

As part of this analysis, we compiled from various references some of the planetary properties during the Phanerozoic that are important to understand the evolution of terrestrial habitability (Figure 2 and 3). They include surface temperature, atmospheric concentrations of oxygen and carbon dioxide, solar luminosity, axial tilt (obliquity), and day length. Atmospheric nitrogen stayed fairly constant (Berner, 2006). The data shown are not the only interpretations and there is considerable variations within some of these estimates by different models and proxies (references in the captions). We used this information to also estimate the mean global surface atmospheric pressure, relative humidity, and atmospheric water concentration during this Eon (Figure 4). Some of these planetary properties were used for our habitability assessments and to put in context our understanding of the Paleo-Earth's habitability.

Figure 2. Mean global surface atmospheric temperature, carbon dioxide and oxygen levels during the Phanerozoic. During this Eon surface temperatures generally decreased by about 8°C with four mayor peaks, CO2 decreased from 7300 ppm to 380 ppm, and O2 was up to 30% during the Permian. Dotted lines represent current values. Temperature data from Royer et al., 2004 assuming a current mean global surface temperature of 15°C. Carbon dioxide levels from Berner et al., 2001 and oxygen levels from Berner, 2009.

Figure 3. Solar luminosity, Earth's axial tilt (obliquity) and day length during the Phanerozoic. During this Eon solar luminosity increased by about 4%, Earth's inclination changed rapidly from 45°to 23°, and day length increased by almost 3 hours. Dotted lines represent current values. Solar luminosity from Caldeira and Kasting, 1992. Obliquity and daylength from Williams, 1993.

Figure 4. Estimated mean global surface atmospheric pressure, relative humidity, and water vapor levels during the Phanerozoic. Atmospheric pressure was mostly influenced by O2 than CO2 levels during this Eon. Earth atmosphere was also much wetter during most of the Phanerozoic.

We calculated the habitability of Earth during the Phanerozoic using two proposed independent methods. The Relative Vegetation Density (RVD) derived from our vegetation datasets of the Visible Paleo-Earth was used as our first proxy for habitability. The RVD is very similar to vegetation indices such as the Normalized Difference Vegetation Index (NDVI) and gives a general idea of the global area-weighted fraction of vegetation cover. Vegetation, as a primary producer, is a good indicator of global habitability, as they provide the resources for many other simple to complex life forms in the trophic scale. Our second habitability indicator was the Standard Primary Habitability (SPH) derived from mean global surface temperatures and relative humidity (Figure 1 and 4). The RVD is a more robust indicator of the actual habitability of a planet as it uses the vegetation cover as a proxy. The SPH is more a measure of climate habitability but it is much easier to estimate.

Both the RVD and the SPH can be used to assess the habitability of Earth-like exoplanets. The RVD requires information about any potential global vegetation cover. This is something very difficult to measure and only possible with future missions (Ford et al., 2001; Seager et al., 2005). The SPH requires two simpler measures, the mean global surface temperature and an estimate of the ocean area coverage. Surface temperature is relatively easier to estimate but ocean area coverage requires extensive light curve observations, still something out of current capabilities (Cowan et al., 2009; Zugger et al., 2010). In any case, these assessments are more focused to landmasses and less can be inferred about their ocean's habitability.

Spectroscopically observations for potential indicators of life (biosignatures), like the combined presence of atmospheric water and oxygen, will probably precede habitability assessments. A puzzling discovery might be the presence of oxygen in an Earth-like exoplanet, a gas associated with the presence of life, in a low habitable environment (i.e. very low temperatures). This suggests (assuming the observations are correct) that the oxygen is produced by a non biogenic process or that some life, as we don't know it, is operating in these conditions. This is why is important to have together measures for the actual presence for life (habitation) and the environment quality (habitability) in observations of exoplanets. Both are necessary to recognize life, even forms distant from terrestrial biology.

Our analysis shows that terrestrial habitability has been greater than today for most of the Phanerozoic as demonstrated by both habitability metrics, RVD and SPH (Figure 5). This makes sense with the fossil record as it is known that life was more abundant during many previous periods and were disturbed by extinction events. Today's terrestrial habitability (SPH) is about 0.6 but it was particularly more habitable during the Devonian (SPH > 0.9) and from the Triassic to the Cretaceous (SPH > 0.7). As expected, a more habitable planet (higher productivity) can support a larger biota (i.e. Dinosaurs). The RVD and SPH are generally correlated after 350 Ma when plants were more common, but there is a notable divergence between 280 Ma to 180 Ma. Both the RVD and SPH show a marked decrease on terrestrial habitability in the last 100 million years.

Luckily, Earth stayed as a mesoplanet during all the Phanerozoic, something that was crucial for the evolution of complex life. Future observations of Earth-like exoplanets may be put in context with the evolution of terrestrial habitability. More analysis will include correlations with the fossil record and other parameters. There are three big questions that need to be answered. First, why terrestrial habitability has been decreasing in the last 100 million years? (if related, the K-Pg mass extinctions was 65 million years ago). Second, what is the future of terrestrial habitability under climate change? Third, what will be the habitability of the first Earth-like exoplanet detected? These are topics for future analysis.

Figure 5. Habitability of Earth during the Phanerozoic as measured by two methods, the Relative Vegetation Density (RVD) and the Standard Primary Habitability (SPH). The RVD is related to the actual abundance of vegetation while the SPH is related to the atmospheric quality for vegetation (productivity). The RVD is not a good indicator in early times as plants started to evolve after 450 Ma, but the SPH shows that Earth was quite habitable in those periods. In general, terrestrial habitability has been about 50% higher than today during most of the Phanerozoic, but it has been steadily decreasing since the last 100 million years.